1. Field of the Invention
The invention is directed to a process and apparatus for heating the process recycle gas utilized in the indirect retorting of oil shale, and more particularly, to a process and apparatus for the combustion of residual carbon in the oil depleted shale to heat the process gas and the recycling of cooled spent shale to moderate the temperature of combustion.
2. Description of the Prior Art
One method of employing a traveling grate retort for processing fresh oil shale requires the circulation of an inert gas through the burden of shale on the grate. Heat is either directly or indirectly provided to recover the oil from oil shale. Vaporized oil from the shale is entrained by the inert gas which is, in turn, conveyed to an oil recovery system. The oil free recycle gas is returned to the traveling grate.
In the direct method of retorting, sufficient oxygen containing air is injected into part of the bed to burn combustible material in the shale, thus raising the temperature of the gas and shale above the oil eduction temperature. The hot gas leaving the shale bed is cooled in heat exchangers to recover the oil before being recycled. The hot spent shale is cooled by the continuous circulation of recycle gas therethrough.
In the indirect method of heating the shale oil burden, an inert gas is heated externally and circulated through the burden to recover the oil. In both the direct and indirect methods of heating, most of the organic carbon produced in the decomposition of the kerogen remains in the spent shale.
It has been suggested that the residual carbon in the spent shale be utilized for the generation of thermal energy. In U.S. Pat. No. 2,434,815, the carbon remaining in spent shale is burned in a vertical shaft furnace, where steam is produced. The steam is used to recover oil from shale. U.S. Pat. No. 2,752,292 teaches the combination of a horizontal conveyor and a vertical retort for recovering oil from shale. In U.S. Pat. No. 4,039,427, oil depleted material containing uncombusted carbons and hydrocarbons is combusted on a separate traveling grate. The combusted hot spent shale is used as a heat source for fresh shale.
In U.S. Pat. No. 4,297,201 the residual carbon in indirectly retorted shale is burned out as the oil depleted shale descends through a vertical kiln which is separate from the retort vessel. Combustion is supported by air flowing countercurrently to the shale with the temperature of combustion controlled by the injection of an inert gas with the air. Additional inert gas is fed into the bottom of the kiln to absorb the heat generated by the combustion process. All of the hot gases are recovered from the top of the vertical kiln and passed through a heat exchanger to heat the process gas used in indirect retorting additional oil shale before being recirculated as the inert gas.